Conventionally, as for inkjet inks, dye inks have been mainly used in view of their high color-developing ability, high reliability, etc.; in recent years, however, attention has also been drawn to pigment inks because the dye inks have disadvantages of poor water resistance and light resistance. In the pigment inks, a polymer compound is used so as to stably disperse a pigment in water or to fix a pigment on a recording medium after the pigment ink is placed on the recording medium (hereinafter, also referred to as attached on a recording medium).
On the other hand, in the inkjet recording apparatus, in order to attain high quality image and high speed printing, the diameters of nozzles serving as an ink jetting unit tend to be reduced. However, a pigment ink containing a polymer compound tends to form aggregation of solid contents due to water evaporation, and when the pigment ink is used in an inkjet recording apparatus equipped with a nozzle having a reduced diameter, ejection stability is hard to be secured. Thus, an attempt is made to improve ejection stability of the pigment ink containing a polymer compound.
Patent Literature 1 discloses an ink of which viscosity is not allowed to increase more than ten-fold when the ink is twice as concentrated. By the use of such ink, the ejection stability is excellent, when an image is recorded on an inkjet paper, a pigment aggregates so as to suppress ink spreading and thus it is possible to prevent occurrence of white spots. However, specific examples of the ink of which viscosity is not allowed to increase more than ten-fold when the ink is twice as concentrated, disclosed in Patent Literature 1 are only inks having a low pigment concentration and a low viscosity. It is difficult to form high-quality images on plain paper using these inks. Moreover, a method for suppressing thickening of the ink having a high pigment concentration upon water evaporation is not disclosed at all.
Patent Literature 2 discloses an ink wherein the rate of increase in viscosity (mPa·s/%) caused by water evaporation of an ink is 5.0 or less when the amount of water evaporation is 30% by mass or less with respect to the total mass of the ink, and the rate of increase in viscosity attains a point (a value) of greater than 50 when the amount of water evaporation is between 30% by mass and 45% by mass. The ink of Patent Literature 2 rapidly thickens upon attaching onto plain paper and drying, thereby exhibiting high printing quality even at high speed printing. However, when the ink dries in a nozzle of a recording apparatus, ejection stability becomes poor.
Patent Literature 3 discloses an ink using a water dispersion of colorant-containing water-insoluble vinyl polymer particles. By the use of the ink, a hydrophobic functional group which inhibits penetration into the inside of paper is included in vinyl polymer particles, so that the vinyl polymer particles easily remain on a paper surface, thereby obtaining high printing density. However, the water-insoluble vinyl polymer particles used in this ink have high aggregation property, causing a problem in ejection stability.
As described above, it has been considered that it is necessary to use an ink of which viscosity rapidly increases due to water evaporation, in order to form a high grade image by printing at high speed on plain paper. However, it is difficult for such ink to secure reliability and handleability, as things stand.
Moreover, a conventional inkjet recording method has a problem that image defects as typified by feathering may be easily caused depending on the combination of an ink and a recording medium, largely decreasing image quality. In order to solve the problem, a method of combining an inkjet ink and a treatment liquid containing fine particles is proposed.
Patent Literature 4 discloses a liquid composition which is applied with an aqueous ink to a recording medium and contains fine particles in a dispersion state, wherein the surfaces of the particles are charged with a polarity opposite to that of the aqueous ink. However, this method is unable to achieve a sufficient effect on the suppression of feathering. Moreover, in these proposals of using the liquid composition containing fine particles, another problem occurs in terms of fixing ability of the recording liquid. Specifically, the liquid composition and the aqueous ink are attached to the recording medium and then a vehicle permeates the recording medium, thereby accumulating a mixture of the fine particles with the colorant on a surface of the recording medium. The thus formed accumulation is mechanically vulnerable, and may be easily removed by rubbing by hand and the like. Therefore, ink smears a user's hand and backface of overlapped ink records, resulting in failure of poor rubfastness.
Patent Literature 5 discloses an inkjet recording method in which a first liquid containing polymer fine particles and an ink composition are attached onto a recording medium, in order to solve the problem of rubfastness. According to the method, as a colorant itself does not have a function for improving rubfastness, a large amount of polymer particles is necessary to obtain a sufficient rubfastness. However, the addition of a large amount of polymer particles may cause side effects, such as decrease in ejection stability and storage stability, viscosity increase or the like. Thus, problems have not been solved yet.